Spectral and luminosity classification for the cool components in symbiotic stars

The near infrared spectra of 12 S-type symbiotic stars and 78 comparison st ars have been observed with moderate dispersion in five runs from 1992 to 1 997, the resolving power being R = lambda/Delta lambda > 2000, with a signa l to noise ratio SIN > 100. The triple-headed absorption band of TiO (lambd a lambda 8432, 8422 and 8452 Angstrom) emerges when a star is later than M2 , and the depth of the TiO absorption band is very sensitive to the spectra l type (ST) and insensitive to the luminosity class of the star. We fit a c urve of spectral type against the index of the absorption depth of this ban d with a standard deviation sigma = 0.37 of a subdivision of one spectral t ype. The IR all triplet (lambda lambda 8498, 8542, 8662 Angstrom); Fe I 868 9 Angstrom: and Fe I 8675 Angstrom are good luminosity indicators although the equivalent widths (EWs) of these lines clearly decrease fora star later than M3. When the star is a supergiant the lines have a smaller central re sidual intensity and broader wings than in the case of a normal giant. The Ca II 8662 Angstrom/Fe I 8675 Pi and Fe I 8689 Angstrom/Fe I 8675 A ratios are also good luminosity indicators for Ii-type giants. The latter is parti cularly useful when there are abundance anomalies. The metal-poor symbiotic star AG Dra is classified as a Ib or II giant, as is TX CVn, on the basis of Fe I 8689 Angstrom/Fe I 8675 Angstrom. 9 other symbiotic stars containin g M-type cool components are classified as giants by direct comparison and quantitative analysis. Due to there being no known good ratio indicator of luminosity for M-type stars in the band studied and because there is no met al abundance data for the symbiotic stars studied by us except for AG Dra, the results for these 9 symbiotic stars are only preliminary. The infrared Ca II triplet of most symbiotic stars clearly varies between the different observing runs. The different luminosity classes given to the same symbioti c star are probably caused by the variability of the lines of ionized eleme nts,: while in some cases they are affected by a low metal abundance.